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DAHDI Telephony Interface Driver

DAHDI stands for Digium Asterisk Hardware Device Interface.

This package contains the kernel modules for DAHDI. For the required userspace tools see the package dahdi-tools.

1. Supported Hardware

1.1. Digital Cards

  • wct4xxp:

    • Digium TE205P/TE207P/TE210P/TE212P: PCI dual-port T1/E1/J1

    • Digium TE405P/TE407P/TE410P/TE412P: PCI quad-port T1/E1/J1

    • Digium TE220: PCI-Express dual-port T1/E1/J1

    • Digium TE420: PCI-Express quad-port T1/E1/J1

    • Digium TE820: PCI-Express eight-port T1/E1/J1

  • wcte12xp:

    • Digium TE120P: PCI single-port T1/E1/J1

    • Digium TE121: PCI-Express single-port T1/E1/J1

    • Digium TE122: PCI single-port T1/E1/J1

  • wcte11xp:

    • Digium TE110P: PCI single-port T1/E1/J1

  • wct1xxp:

    • Digium T100P: PCI single-port T1

    • Digium E100P: PCI single-port E1

  • wcb4xxp:

    • Digium B410: PCI quad-port BRI

  • tor2: Tormenta quad-span T1/E1 card from the Zapata Telephony project

1.2. Analog Cards

  • wctdm24xxp:

    • Digium TDM2400P/AEX2400: up to 24 analog ports

    • Digium TDM800P/AEX800: up to 8 analog ports

    • Digium TDM410P/AEX410: up to 4 analog ports

    • Digium Hx8 Series: Up to 8 analog or BRI ports

  • wctdm:

    • Digium TDM400P: up to 4 analog ports

  • xpp: Xorcom Astribank: a USB connected unit of up to 32 ports (including the digital BRI and E1/T1 modules)

  • wcfxo: X100P, similar and clones. A simple single-port FXO card

1.3. Other Drivers

  • pciradio: Zapata Telephony PCI Quad Radio Interface

  • wctc4xxp: Digium hardware transcoder cards (also need dahdi_transcode)

  • dahdi_dynamic_eth: TDM over Ethernet (TDMoE) driver. Requires dahdi_dynamic

  • dahdi_dynamic_loc: Mirror a local span. Requires dahdi_dynamic

2. Installation

If all is well, you just need to run the following:

make install

You’ll need the utilities provided in the package dahdi-tools to configure DAHDI devices on your system.

If using sudo to build/install, you may need to add /sbin to your PATH.

If you still have problems, read further.

2.1. Build Requirements

gcc and friends. Generally you will need to install the package gcc. There may be cases where you will need a specific version of gcc to build kernel modules.

2.1.1. Kernel Source / "Headers"

  • Building DAHDI-linux requires a kernel build tree.

  • This should basically be at least a partial kernel source tree and most importantly, the exact kernel .config file used for the build as well as several files generated at kernel build time.

  • KERNEL_VERSION is the output of the command uname -r

  • If you build your own kernel, you need to point to the exact kernel build tree. Luckily for you, this will typically be pointed by the symbolic link /lib/modules/KERNEL_VERSION/build which is the location zaptel checks by default.

  • If you use a kernel from your distribution you will typically have a package with all the files required to build a kernel modules for your kernel image.

    • On Debian and Ubuntu this is linux-headers-`uname -r`

    • On Fedora, RHEL and compatibles (e.g. CentOS) and SUSE this is the kernel-devel package. Or if you run kernel-smp or kernel-xen, you need kernel-smp-devel or kernel-xen-devel, respectively.

    • In some distributions (e.g.: in RHEL/CentOS, Fedora, Ubuntu) the installation of the kernel-devel / kernel-headers package will be of a version that is newer than the one you currently run. In such a case you may need to upgrade the kernel package itself as well and reboot.

  • To point explicitly to a different build tree: set KSRC to the kernel source tree or KVERS to the exact kernel version (if "headers" are available for a different version). This parameter must be run on every calls to make (e.g.: make clean, make install).

    make KVERS=2.6.18.Custom
    make KSRC=/home/tzafrir/kernels/linus

2.1.2. Kernel Configuration

If you build a custom kernel, note the following configuration items:

  • CONFIG_CRC_CCITT must be enabled (y or m). On 2.6 kernels this can be selected These can be selected from the "Library Routines" submenu during kernel configuration via "make menuconfig".

  • DAHDI will work if you disable module unloading. But you may need extra reboots.

  • DAHDI needs the BKL (Big Kernel Lock). This may be annoying in >=2.6.37 kernels. Make sure you enable CONFIG_BKL on those kernels.

2.2. Installing to a Subtree

The following may be useful when testing the package or when preparing a package for a binary distribution (such as an rpm package) installing onto a subtree rather than on the real system.

make install DESTDIR=targetdir

This can be useful for any partial install target of the above (e.g: install-modules or install-programs).

the targetdir must be an absolute path, at least if you install the modules. To install to a relative path you can use something like:

make install-modules DESTDIR=$PWD/target

2.3. Extra Modules

To build extra modules / modules directory not included in the DAHDI distribution, use the optional variables MODULES_EXTRA and SUBDIRS_EXTRA:

make MODULES_EXTRA="mod1 mod2"
make MODULES_EXTRA="mod1 mod2" SUBDIRS_EXTRA="subdir1/ subdir1/"

2.4. Static Device Files

Userspace programs communicate with the DAHDI modules through special device files under /dev/dahdi . Those are normally created by udev, but if you use an embedded-type system and don’t wish to use udev, you can generate them with the following script, from the source directory:


This will generate the device files under /dev/dahdi . If you need to generate them elsewhere (e.g. tmp/newroot/dev/dahdi) use the option -d to the script:

./build_tools/make_static_devs -d tmp/newroot/dev/dahdi

2.5. DKMS

DKMS, Dynamic Kernel Module Support, is a framework for building Linux kernel modules. It is used, among others, by several distributions that package the DAHDI kernel modules.

DKMS is designed to provide updates over drivers installed from original kernel modules tree. Thus it installed modules into /lib/modules/updates or /lib/modules/VERSION/updates . This is generally not an issue on normal operation. However if you try to install DAHDI from source on a system with DAHDI installed from DKMS this way (potentially of an older version), be sure to remove the DKMS-installed modules from the updates directory. If you’re not sure, the following command will give you a clue of the versions installed:

find /lib/modules -name dahdi.ko

2.6. Installing the B410P drivers with mISDN

DAHDI includes the wcb4xxp driver for the B410P, however, support for the B410P was historically provided by mISDN. If you would like to use the mISDN driver with the B410P, please comment out the wcb4xxp line in /etc/dahdi/modules. This will prevent DAHDI from loading wcb4xxp which will conflict with the mISDN driver.

To install the mISDN driver for the B410P, please see for more information, but the following sequence of steps is roughly equivalent to make b410p from previous releases.

tar xfz mISDN-1_1_8.tar.gz
tar xfz mISDNuser-1_1_8.tar.gz
pushd mISDN-1_1_8
make install
pushd mISDNuser-1_1_8
make install
/usr/sbin/misdn-init config

You will then also want to make sure /etc/init.d/misdn-init is started automatically with either chkconfig --add misdn-init or update-rc.d misdn-init defaults 15 30 depending on your distribution.

At the time this was written, misdn-1.1.8 is not compatible the 2.6.25 kernel. Please use a kernel version 2.6.25 or earlier.

2.7. OSLEC

OSLEC is an Open Source Line Echo Canceller. It is currently in the staging subtree of the mainline kernel and will hopefully be fully merged at around version 2.6.29. The echo canceller module dahdi_echocan_oslec provides a DAHDI echo canceller module that uses the code from OSLEC. As OSLEC has not been accepted into mainline yet, its interface is not set in stone and thus this driver may need to change. Thus it is not built by default.

Luckily the structure of the dahdi-linux tree matches that of the kernel tree. Hence you can basically copy drivers/staging/echo and place it under driver/staging/echo . In fact, dahdi_echocan_oslec assumes that this is where the oslec code lies. If it is elsewhere you’ll need to fix the #include line.

Thus for the moment, the simplest way to build OSLEC with dahdi is to copy the directory drivers/staging/echo from a recent kernel tree (at least 2.6.28-rc1) to the a subdirectory with the same name in the dahdi-linux tree.

After doing that, you’ll see the following when building (running make)

CC [M] /home/tzafrir/dahdi-linux/drivers/dahdi/dahdi_echocan_oslec.o
CC [M] /home/tzafrir/dahdi-linux/drivers/dahdi/../staging/echo/echo.o

As this is an experimental driver, problems building and using it should be reported on the OSLEC mailing list.

Alternatively you can also get the OSLEC code from the dahdi-linux-extra GIT repository:

git clone git://
cd dahdi-linux-extra
git archive extra-2.6 drivers/staging | (cd ..; tar xf -)
cd ..; rm -rf dahdi-linux-extra

2.8. Live Install

In many cases you already have DAHDI installed on your system but would like to try a different version. E.g. in order to check if the latest version fixes a bug that your current system happens to have.

DAHDI-linux includes a script to automate the task of installing DAHDI to a subtree and using it instead of the system copy. Module loading through modprobe cannot be used. Thus the script pre-loads the required modules with insmod (which requires some quesswork as for which modules to load). It also sets PATH and other environment variables to make all the commands do the right thing.

There is an extra mode of operation to copy all the required files to a remote host and run things there, for those who don’t like to test code on thir build system.

2.8.1. Live Install: The Basics

Basic operation is through running


from the root directory of the dahdi-linux tree. Using DAHDI requires dahdi-tools as well, and the script builds and installs dahdi-tools. By default it assumes the tree of dahdi-tools is in the directory dahdi-tools alongside the dahdi-linux tree. If you want to checkout the trunks from SVN, use:

svn checkout dahdi-linux
svn checkout dahdi-tools
cd dahdi-linux

If the tools directory resides elsewhere, you’ll need to edit live/live.conf (see later on). The usage message of live_dahdi:

Usage:                    equivalent of:
live_dahdi configure      ./configure
live_dahdi install        make install
live_dahdi config         make config
live_dahdi unload         /etc/init.d/dahdi stop
live_dahdi load           /etc/init.d/dahdi start
live_dahdi reload         /etc/init.d/dahdi restart
live_dahdi xpp-firm       (Reset and load xpp firmware)
live_dahdi rsync TARGET   (copy filea to /tmp/live in host TARGET)
live_dahdi exec  COMMAND  (Run COMMAND in 'live' environment)

Normally you should run:

./build_tools/live_dahdi configure
./build_tools/live_dahdi install
./build_tools/live_dahdi config

to build and install everything. Up until now no real change was done. This could actually be run by a non-root user. All files are installed under the subdirectory live/ .

Reloading the modules (and restarting Asterisk) is done by:

./build_tools/live_dahdi reload

Note: this stops Asterisk, unloads the DAHDI modules, loads the DAHDI modules from the live/ subdirectory, configures the system and re-starts Asterisk. This can do damage to your system. Furthermore, the DAHDI configuration is generated by dahdi_genconf. It can be influenced by a genconf_parameters file. But it may or may not be what you want.

If you want to run a command in the environment of the live system, use the command exec:

./build_tools/live_dahdi lsdahdi
./build_tools/live_dahdi dahdi_hardware -v

Note however:

./build_tools/live_dahdi dahdi_cfg -c live/etc/dahdi/system.conf

2.8.2. Live Install Remote

As mentioned above, live_dahdi can also copy all the live system files to a remote system and run from there. This requires rsync installed on both system and assumes you can connect to the remove system through ssh.

tzafrir@hilbert $ ./build_tools/live_dahdi rsync root@david
root@david's password:
<f+++++++++ live_dahdi
cd+++++++++ live/
<f+++++++++ live/live.conf
cd+++++++++ live/dev/
cd+++++++++ live/dev/dahdi/
cd+++++++++ live/etc/
cd+++++++++ live/etc/asterisk/
cd+++++++++ live/etc/dahdi/
<f+++++++++ live/etc/dahdi/genconf_parameters
<f+++++++++ live/etc/dahdi/init.conf

As you can see, it copies the script itselfand the whole live/ subdirectory. The target directory is /tmp/live on the target directory (changing it should probably be simple, but I never needed that).

Then, on the remove computer:

root@david:/tmp# ./live_dahdi reload

2.8.3. Configuring a Live Install

The live_dahdi script reads a configuration file in live/live.conf if it exists. This file has the format of a shell script snippet:

var1=value # a '#' sign begins a comment
# comments and empty lines are ignored

The variables below can also be overriden from the environment:

var1='value' ./build_tools/live_dahdi

The relative path to the dahdi-linux tree. The default is . and normally there’s no reason to override it.


The relative path to the dahdi-tools tree. The default is dahdi-tools.


Set a syncing astribank. See xpp_sync(8). Default is auto.


The command for an init.d script to start/stop Asterisk. Should accept start and stop commands. Use true if you want to make that a no-op. Defaults to /etc/init.d/asterisk .


Manual list of modules. They will be loaded by insmod. If reside in a subdir, add it explicitly. Modules for the drivers that are detected on the system will be added by the script. Default: dahdi dahdi_echocan_mg2


A list of modules to remove when unloading. Will be resolved recursively. The default is dahdi. You may also want to have oslec or hpec there if you use them.


If you want to build DAHDI for a different kernel version than the one currently running on the system (mostly useful for a remote install). Note that you will normally need to export this, in order for this to take effect on the make command. In live/live.conf itself have the line:

export KVERS="2.6.39-local"

Alternatively, if you want to point to an exact kernel source tree, set it with KSRC. Just like KVERS above, it needs to be explicitly exported.

export KSRC="/usr/src/tree/linux"

3. Module Parameters

The kernel modules can be configured through module parameters. Module parameters can optionally be set at load time. They are normally set (if needed) by a line in a file under /etc/modprobe.d/ or in the file /etc/modprobe.conf.

Example line:

options dahdi debug=1

The module parameters can normally be modified at runtime through sysfs:

pungenday:~# cat /sys/module/dahdi/parameters/debug
pungenday:~# echo 1 >/sys/module/dahdi/parameters/debug
pungenday:~# cat /sys/module/dahdi/parameters/debug

Viewing and setting parameters that way is possible as of kernel 2.6 . In kernels older than 2.6.10, the sysfs "files" for the parameters reside directly under /sys/module/module_name .

Useful module parameters:

3.1. debug

(most modules)

Sets debug mode / debug level. With most modules debug can be either disabled (0, the default value) or enabled (any other value).

wctdm and wcte1xp print several extra debugging messages if the value of debug is more than 1.

Some modules have "debugging flags" bits - the value of debug is a bitmask and several messages are printed if some bits are set:

To get a list of parameters supported by a module, use

modinfo module_name

Or, for a module you have just built:

modinfo ./drivers/dahdi/module_name.ko

For the xpp modules this will also include the description and default value of the module. You can find a list of useful xpp module parameters in README.Astribank .

  • wctdm24xxp:

    • 1: DEBUG_CARD


  • wct4xxp:

    • 1: DEBUG_MAIN

    • 2: DEBUG_DTMF

    • 4: DEBUG_REGS

    • 8: DEBUG_TSI


    • 32: DEBUG_RBS

    • 64: DEBUG_FRAMER

  • xpp: See also README.Astribank:

    • 1: GENERAL - General debug comments.

    • 2: PCM - PCM-related messages. Tend to flood logs.

    • 4: LEDS - Anything related to the LEDs status control. The driver produces a lot of messages when the option is enabled.

    • 8: SYNC - Synchronization related messages.

    • 16: SIGNAL - DAHDI signalling related messages.

    • 32: PROC - Messages related to the procfs interface.

    • 64: REGS - Reading and writing to chip registers. Tends to flood logs.

    • 128: DEVICES - Device instantiation, destruction and such.

    • 256 - COMMANDS - Protocol commands. Tends to flood logs.

      The script xpp_debug in the source tree can help settting them at run time.

3.2. deftaps


The default size for the echo canceller. The number is in "taps", that is "samples", 1/8 ms. The default is 64 - for a tail size of 8 ms.

Asterisk’s chan_dahdi tends to pass its own value anyway, with a different default size. So normally setting this doesn’t change anything.

3.3. max_pseudo_channels


The maximum number of pseudo channels that dahdi will allow userspace to create. Pseudo channels are used when conferencing channels together. The default is 512.

3.4. auto_assign_spans


See Span Assignments below.

3.5. XPP (Astribank) module parameters

3.5.1. debug

(all modules) - see above.

3.5.2. dahdi_autoreg


Deprecated. See dahdi.auto_assign_spans above.

Originally had a somewhat similar (but xpp-specific and more limited) role to auto_assign_spans. For backward compatibility this variable is still kept, but its value is unused. Astribanks will auto-register with dahdi if auto_assign_spans is not set.

3.5.3. tools_rootdir


Defaults to /. Passed (as the variable DAHDI_TOOLS_ROOTDIR) to generated events (which can be used in udev hooks). Also serves as the base of the variable DAHDI_INIT_DIR (by default: $DAHDI_TOOLS_DIR/usr/share/dahdi).

3.5.4. initdir


Deprecated. Setting both initdir and tools_rootdir will generate an error.

A directory under tools_rootdir containing the initialization scripts. The default is /usr/share/dahdi . Setting this value could be useful if that location is inconvenient for you.

3.5.5. rx_tasklet


Enable (1) or disable (0) doing most of the packets processing in separate tasklets. This should probably help on higher-end systems with multiple Astribanks.

3.5.6. vmwi_ioctl


Does userspace support VMWI notification via ioctl? Default: 1 (yes).

Disable this (0) to have the driver attempt to detect the voicemail message waiting indication status for this port from FSK messages userspace (Asterisk) sends. Set the ports to use AC neon-lamp style message waiting indication. The detection from the FSK messages takes extra CPU cycles but is required with e.g. Asterisk < 1.6.0 .

Also note that in order for this parameter to take effect, it must be set before the span is registered. This practically means that it should be set through modprobe.d files.

See also Voicemail Indication in README.Astribank.

3.5.7. usb1


Enable (1) or disable (0) support of USB1 devices. Disabled by default.

USB1 devices are not well-tested. It seems that they don’t work at all for Astribank BRI. Generally they should work with the current code, but we expect the voice quality issues. Hence we would like to make it very clear that you if you have a USB1 port (rather than a USB2 one, as recommended) you will have to take an action to enable the device.

3.5.8. poll intervals


There are various values which the driver occasionally polls the device for. For instance, the parameter poll_battery_interval for xpd_fxo to poll the battery, in order to know if the telco line is actually connected.

The value of those parameters is typically a number in milliseconds. 0 is used to disable polling. Under normal operation there should be no reason to play with those parameters.

3.5.9. dtmf_detection


Enable (1) or disable (0) support of hardware DTMF detection by the Astribank.

3.5.10. caller_id_style


Various types of caller ID signalling styles require knowing the PCM even when the line is on-hook (which is usually a waste of CPU and bandwidth). This parameter allows fine-tuning the behaviour here:

  • 0 (default) - Don’t pass extra PCM when on-hook.

  • 1 ETSI-FSK: Wait for polarity reversal to come before a ring and then start passing PCM until the caller ID has been passed.

  • 2 ETSI-DTMF: Always pass PCM and generate a DTMF if polarity reversal is detected before ring.

  • 3 Passthrough: Always pass PCM as-is.

This parameter is read-only. It cannot be changed at run-time.

3.5.11. battery_threshold


Minimum voltage that shows there is battery. Defaults to 3. Normally you should not need to change this, unless dealing with a funky PSTN provider.

3.5.12. battery_debounce


Minimum interval (msec) for detection of battery off (as opposed to e.g. a temporary power denial to signal a hangup). Defaults to 1000. As with battery_threshold above, there’s normally no need to tweak it.

3.5.13. use_polrev_firmware


Enable (1, default) or disable (0) support for polarity reversal detection in the hardware. Only has effect with PIC_TYPE_2.hex rev. >= 11039 and with the initialization changes (init_card_2_30) in rev. 949aa49.

This parameter is read-only. It cannot be changed at run-time.

4. Internals

4.1. DAHDI Device Files

Userspace programs will usually interact with DAHDI through device files under the /dev/dahdi directory (pedantically: character device files with major number 196) . Those device files can be generated statically or dynamically through the udev system.

  • /dev/dahdi/ctl (196:0) - a general device file for various information and control operations on the DAHDI channels.

  • /dev/dahdi/chan/N/M - A device file for channel M in span N

    • Both N and M are zero padded 3 digit numbers

    • Both N and M start at 001

    • M is chanpos - numbering relative to the current span.

  • /dev/dahdi/NNN (196:NNN) - for NNN in the range 1-249. A device file for DAHDI channel NNN. It can be used to read data from the channel and write data to the channel. It is not generated by default but may be generated as a symlink using udev rules.

  • /dev/dahdi/transcode (196:250) - Used to connect to a DAHDI transcoding device.

  • /dev/dahdi/timer (196:253) - Allows setting timers. Used anywhere?

  • /dev/dahdi/channel (196:254) - Can be used to open an arbitrary DAHDI channel. This is an alternative to /dev/dahdi/NNN that is not limited to 249 channels.

  • /dev/dahdi/pseudo (196:255) - A timing-only device. Every time you open it, a new DAHDI channel is created. That DAHDI channel is "pseudo" - DAHDI receives no data in it, and only sends garbage data with the same timing as the DAHDI timing master device.

4.2. DAHDI Timing

A PBX system should generally have a single clock. If you are connected to a telephony provider via a digital interface (e.g: E1, T1) you should also typically use the provider’s clock (as you get through the interface). Hence one important job of Asterisk is to provide timing to the PBX.

DAHDI "ticks" once per millisecond (1000 times per second). On each tick every active DAHDI channel reads and 8 bytes of data. Asterisk also uses this for timing, through a DAHDI pseudo channel it opens.

However, not all PBX systems are connected to a telephony provider via a T1 or similar connection. With an analog connection you are not synced to the other party. And some systems don’t have DAHDI hardware at all. Even a digital card may be used for other uses or is simply not connected to a provider. DAHDI cards are also capable of providing timing from a clock on card. Cheap x100P clone cards are sometimes used for that purpose.

If a hardware timing source either cannot be found or stops providing timing during runtime, DAHDI will automatically use the host timer in order provide timing.

You can check the DAHDI timing source with dahdi_test, which is a small utility that is included with DAHDI. It runs in cycles. In each such cycle it tries to read 8192 bytes, and sees how long it takes. If DAHDI is not loaded or you don’t have the device files, it will fail immediately. If you lack a timing device it will hang forever in the first cycle. Otherwise it will just give you in each cycle the percent of how close it was. Also try running it with the option -v for a verbose output.

4.3. Spans and Channels

DAHDI provides telephony channels to the userspace applications. Those channels are channels are incorporated into logical units called spans.

With digital telephony adapters (e.g: E1 or T1), a span normally represents a single port. With analog telephony a span typically represents a PCI adapter or a similar logical unit.

Both channels and spans are identified by enumerating numbers (beginning with 1). The number of the channel is the lowest unused one when it is generated, and ditto for spans.

There are up to 128 spans and 1024 channels. This is a hard-wired limit (see dahdi/user.h . Several places throuout the code assume a channel number fits in a 16 bits number). Channel and span numbers start at 1.

4.4. Span Assignments

A DAHDI device (e.g. a PCI card) is represented within the DAHDI drivers as a DAHDI device. Normally (with auto_assign_spans=1 in the module dahdi, which is the default), when a device is discovered and loaded, it registers with the DAHDI core and its spans automatically become available. However if you have more than one device, you may be interested to set explicit spans and channels numbers for them. To use manual span assigment, set auto_assign_spans to 0 . e.g. in a file under /etc/modprobe.d/ include the following line:

options dahdi auto_assign_spans=0

You will then need to assign the spans manually at device startup. You will need to assign a span number and channel numbers for each available span on the system. On my test system I have one BRI PCI card and one Astribank BRI+FXS:

# grep . /sys/bus/dahdi_devices/devices/*/spantype

All spans here, except the FXS one, are BRI spans with 3 channels per span.

In order to assign a span, we write three numbers separated by colns to the file assign_span in the SysFS node



echo 9:5:10 >/sys/bus/dahdi_devices/devices/astribanks:xbus-00/assign_span
echo 2:8:40 >/sys/bus/dahdi_devices/devices/pci:0000:00:09.0/assign_span
echo 1:1:1  >/sys/bus/dahdi_devices/devices/astribanks:xbus-00/assign_span
echo 4:6:20 >/sys/bus/dahdi_devices/devices/pci:0000:00:09.0/assign_span
echo 3:2:5  >/sys/bus/dahdi_devices/devices/astribanks:xbus-00/assign_span

As you can see, the order of span numbers or local span number is insignificant. However the order of channel numbers must be the same as that of span numbers.

Which indeed produced:

# head -n3 -q /proc/dahdi/*
Span 1: XBUS-00/XPD-00 "Xorcom XPD [usb:LAB-0003].1: BRI_NT"
           1 XPP_BRI_NT/00/00/0
Span 2: XBUS-00/XPD-02 "Xorcom XPD [usb:LAB-0003].3: BRI_TE"
           5 XPP_BRI_TE/00/02/0
Span 5: XBUS-00/XPD-10 "Xorcom XPD [usb:LAB-0003].9: FXS" (MASTER)
          10 XPP_FXS/00/10/0
Span 6: B4/0/4 "B4XXP (PCI) Card 0 Span 4" RED
          23 B4/0/4/1 YELLOW
Span 8: B4/0/2 "B4XXP (PCI) Card 0 Span 2" RED
40 B4/0/2/1 RED

Likewise spans can be unassigned by writing to the unassign-span "file".

4.5. Dynamic Spans

Dynamic spans are spans that are not represented by real hardware. Currently there are two types of them:


TDM over Ethernet. A remote span is identified by an ethernet (MAC) address.


Generates a span that is actually a loopback to a different local span.

Modules that support the dynamic spans are typically loaded at the time the ioctl DAHDI_DYNAMIC_CREATE is called. This is typically called by dahdi_cfg when it has a line such as:


in /etc/dahdi/system.conf . In that case it will typically try to load (through modprobe) the modules dahdi_dynamic and dahdi_dynamic_’somename'. It will then pass params to it.

Dynamic spans are known to be tricky and are some of the least-tested parts of DAHDI.

4.6. Echo Cancellers

(To be documented later)

4.7. Tone Zones

(To be documented later)

4.8. PROCFS Interface: /proc/dahdi

A simple way to get the current list of spans and channels each span contains is the files under /proc/dahdi . /proc/dahdi is generated by DAHDI as it loads. As each span registers to DAHDI, a file under /proc/dahdi is created for it. The name of that file is the number of that span.

Each file has a 1-line title for the span followed by a few optional general counter lines, an empty line and then a line for each channel of the span.

The title line shows the number of the span, its name and title, and (potentially) the alarms in which it is.

The title shows the span number and name, followed by any alarms the span may have: For example, here is the first span in my system (with no alarms):

Span 1: XBUS-00/XPD-00 "Xorcom XPD #0/0: FXS"

There are several extra optional keywords that may be added there:


This span is the master span. See DAHDI Timing.


The clock source among several spans that belong to a single E1/J1/T1 card.


The span is in alarm

Following that line there may be some optional lines about IRQ misses, timing slips and such, if there are any.

The channel line for each channel shows its channel number, name and the actual signalling assigned to it through dahdi_cfg. Before being configured by dahdi_cfg: This is DAHDI channel 2, whose name is XPP_FXS/0/0/1.

2 XPP_FXS/0/0/1

After being configured by dahdi_cfg: the signalling FXOLS was added. FXS channels have FXO signalling and vice versa:


If the channel is in use (typically opened by Asterisk) then you will see an extra (In use):

2 XPP_FXS/0/0/1 FXOLS (In use)

4.9. SysFS Interface

DAHDI exposes several interfaces under the SysFS virtual file system. SysFS represents kernel objects in nodes - directories. There properties are often files. They may also contain other nodes or include symlinks to other nodes.

4.9.1. Class DAHDI

Under /sys/class/dadhi there exists a node for the non-channel DAHDI device file under /dev/dahdi. The name is dahdi!foo for the file /dev/dahdi/foo (udev translates exclamation marks to slashes). Those nodes are not, for the most part, proper SysFS nodes, and don’t include any interesting properties. The files in this class ctl, timer, channel, pseudo and (if exists) transcode.

4.9.2. Devices Bus

Each DAHDI device (a physical device, such as a PCI card) is represented by a node under /sys/bus/dahdi_devices/devices named with the name of its device.

Its attributes include:


Write-only attribute: this device’s spans should now be assigned ("registered"). See section about [_span_assignments].


Write-only attribute. Spans in the device auto-assign ("register" as in the original interface). See section about [_span_assignments].


A unique hardware-level identifier (e.g. serial number), if available.


The name of the manufacturer. Freeform-string.


The time at which the device registered with the DAHDI core. Example value: "0005634136.941901429".


A line for each available span: <num>:<type>. This has to be provided here as in the case of manual assignment, userspace may need to know it before span nodes are created.


Device type.


Write-only attribute: the span whose device-local number is written should now be unassigned ("unregistered"). See section about [_span_assignments].

4.9.3. Spans Bus

Each DAHDI span is represented by a node under /sys/bus/dahdi_spans/devices with the name span-N (where N is the number of the span). Spans of each device also reside under the node of the device.

Useful attributes in the span node:


The alarms of the span. Currently this is a numeric representation. This may change in the future.


The channel number of the first channel. The channel numbers of the following channels are guaranteed to follow it.


The number of the channels in the span.


A free-form description of the span.


1 if the span is digital, 0 if it isn’t.


1 if the span is the sync master, 0 if it isn’t.


LBO setting for the channel.


The framing and coding of the span, for a digital span. Textual represenation:



The number of the span within the DAHDI device.


A concise name for this span.


A very short type string.


Current sync source.

4.9.4. sys/bus/dahdi_spans/drivers/generic_lowlevel/master_span

All spans in the bus are handled by a single driver. The driver has one non-standard attribute: master_span. printing it shows the current DAHDI master span writing a number to it forces setting this span as the master span.

4.9.5. Channels Bus

Each DAHDI channel is represented by a node under /sys/bus/dahdi_channels/devices with the name dahdi!channels!N!M (where N is the number of the span and M is the number of the channel in the span - chanpos). Channels of each span also reside under the node of the span.

Useful attributes in the channel node (All attributed listed below are read-only):


List of names of the current active alarms (space separated). Normally (no alarms) empty. Example:


The block size set by userspace.


The (global) channel number.


The channel number within the span.


Major and minor device numbers.


The name of the echo canceller to be used in the channel, if one is configured. Example:


State of the echo canceller. ACTIVE: configured and inuse. INACTIVE otherwise.


1 if the channel is in use (was opepend by userspace), 0 otherwise.


A name string for the channel


The signalling types set for the channel. A space-separated list of signalling types.


The signalling types this channel may be configured to handle. A space- separated list of signalling types.

4.10. User-space Interface

User-space programs can only work with DAHDI channels. The basic operation is read() to read audio from the device and write() to write audio to it. Audio can be encoded as either alaw (G.711a) or (m)ulaw (G.711u). See the ioctl DAHDI_SETLAW.

While it is technically possible to use /dev/dahdi/NUMBER directly, this will only work for channel numbers up to 249. Generally you should use:

int channo = CHAN_NUMBER_TO_OPEN;
int rc;
int fd = open("/dev/dahdi/channel", O_RDRW, 0600);
// Make sure fd >= 0
rc = ioctl(fd, DAHDI_SPECIFY, &channo) < 0);
// Make sure this rc >= 0

FIXME: there’s more to tell about the user-space interface.

4.11. Configuration

Most of the configuration is applied from userspace using the tool dahdi_cfg in the package dahdi_tools. This section will not cover the specifics of that file. Rather it will cover the way configuration from this file is applied. Also note that there are other methods to configure DAHDI drivers: there are Module Parameters. The xpp driver have their own separate initialization scripts and xpp.conf that arecovered in README.Astribank.

When a span is registered, it is considered "unconfigured". Only after dahdi_cfg has been run to apply configuration, the span is ready to run.

Some of the configuration is handled by the DAHDI core. Some of it is handled by callbacks, which are function pointers in the ‘struct dahdi_span’: spanconfig, chanconfig and (in a way) startup.

Dahdi_cfg starts by reading the configuration from the configuration file (/etc/dahdi/system.conf, by default), and creating a local configuration to apply. If you use -v, at this stage it will pront the configuration that is "about to be configured". Then it will start to actually apply the configuration.

Before actually applying configuration, it destroys any existing dynamic spans and generates new ones (if so configured. FIXME: what about running DAHDI_SPANCONFIG for new dynamic spans?).

Next thing it will do is apply the parameters from span lines using the ioctl DAHDI_SPANCONFIG. Some generic processing of parameters passed in DAHDI_SPANCONFIG is done in the DAHDI core, in the callback function spanconfig in , but most of it is left to spanconfig callback of the span (if it exists. This one typically does not exists on analog cards).

Now dahdi_cfg will ask each existing channel for its existing configuration and apply configuration if configuration changes are needed. Configuration is applied to the channels using the ioctl call DAHDI_CHANCONFIG ioctl. As in the case of the span configuration, part of it is applied by the DAHDI core, and then it is handed over to the span’s chanconfig callback. Typically all spans will have such a callback.

Echo cancellers and tone-zones are handled separately later.

Once everything is done, the ioctl DAHDI_STARTUP is called for every span. This is also translated to a call to the optional span callback startup.

Finally the ioctl DAHDI_HDLC_RATE is called for every channel (that is: if 56k is not set for the channel, it will be explicitly set to the standard HDLC rate of 64k).

4.12. Low-Level Drivers

Low-level drivers create spans (struct dahdi_span). They register the spans with the DAHDI core using dahdi_device_register().

struct dahdi_span has a number of informative members that are updated solely by the low-level driver:


A concise span name. E.g.: Foo/1


A slightly longer span name.


Span type in text form.


Span’s device manufacturer


Span’s device type


span device’s location in system


IRQ for this span’s hardware


Interrupt misses


Clock slips

There are various function pointers in the struct dahdi_span which are used by the DAHDI core to call the low-level driver. Most of them are optional.

The following apply to a span:


FIXME: seems to be unused.


Basic span configuration (called from dahdi_cfg).


Last minute initialization after the configuration was applied.


Explicit shutdown (e.g. for dynamic spans). Normally not needed.


Enable/disable maintinance mode (FIXME: document).


Get notified that the master span has ticked.

The following apply to a single channel.


Configure the channel (called from dahdi_cfg).


Channel was opened for read/write from user-space.


Channel was closed by user-space.


Handle extra ioctls. Should return -ENOTTY if ioctl is not known to the channel


Create a custom echo canceller. Normally used for providing a hardware echo canceller. If NULL, the standard DAHDI echo canceller modules will be used.


Copy signalling bits to device. See below on signalling.


Implement RBS-like signalling-handling. See below on signalling.


Handle signalling yourself. See below on signalling.


Used to tell an onboard HDLC controller that there is data ready to transmit.


(if DAHDI_AUDIO_NOTIFY is set) - be notified when the channel is (or isn’t) in audio mode. Which may mean (for an ISDN B-channel) that its data need not be sent.

There are several alternative methods for a span to use for signalling. One of them should be used.

4.12.1. Signalling: rbsbits

If the device is a CAS interface, the driver should copy the signalling bits to and from the other side, and DAHDI will handle the signalling. The driver just need to provide a rbsbits and set DAHDI_FLAG_RBS in the span→flags.

Note that rbs (Robed Bits Signalling) here merely refers to the (up to) 4 signalling bits of the channel. In T1 they are transmitted by "robbing" bits from the channels and hence the name. In E1 they are transmitted in a timeframe of their own.

The driver should then signal a change in the signalling bits in a channel using dahdi_rbsbits().

4.12.2. Signalling: hooksig

If the device does not know about signalling bits, but has their equivalents (i.e. can disconnect battery, detect off hook, generate ring, etc directly) then the driver can specify a sethook function and set DAHDI_FLAG_RBS in span→flags. In that case DAHDI will call that function whenever the signalling state changes.

The hooksig function is only used if the rbsbits function is not set.

The span should notify DAHDI of a change of signalling in a channel using dahdi_hooksig().

4.12.3. Signalling: sethook

Alternatively, if DAHDI_FLAG_RBS is not set in the flags of the span (to use either rbsbits or hooksig), the DAHDI core will try to call the sethook function of the span (if it exists) to handle individual hook states.

The span should then notify DAHDI of a change in the signalling state using dahdi_sethook().

FIXME: anybody using this one?

4.13. ABI Compatibility

Like any other kernel code, DAHDI strives to maintain a stable interface to userspace programs. The API of DAHDI to userspace programs, dahdi/user.h, has remained backward-compatible for a long time and is expected to remain so in the future. With the ABI (the bits themselves) things are slightly trickier.

DAHDI’s interface to userspace is mostly ioctl(3) calls. Ioctl calls are identified by a number that stems from various things, one of which is the size of the data structure passed between the kernel and userspace.

Many of the DAHDI ioctl-s use some specific structs to pass information between kernel and userspace. In some cases the need arose to pass a few more data members in each call. Simply adding a new member to the struct would have meant a new number for the ioctl, as its number depends on the size of the data passed.

Thus we would add a new ioctl with the same base number and with the original struct.

So suppose we had the following ioctl:

struct dahdi_example {
        int sample;

#define DAHDI_EXAMPLE     _IOWR (DAHDI_CODE, 62, struct dahdi_example)

And we want to add the field int onemore, we won’t just add it to the struct. We will do something that is more complex:

/* The original, unchanged: */
struct dahdi_example_v1 {
        int sample;

/* The new struct: */
struct dahdi_example {
        int sample;
        int onemore;

#define DAHDI_EXAMPLE_V1  _IOWR(DAHDI_CODE, 62, struct dahdi_example_v1)
#define DAHDI_EXAMPLE     _IOWR(DAHDI_CODE, 62, struct dahdi_example)

We actually have here two different ioctls: the old DAHDI_EXAMPLE would be 0xC004DA3E . DAHDI_EXAMPLE_V1 would have the same value. But the new value of DAHDI_EXAMPLE would be 0xC008DA3E .

Programs built with the original dahdi/user.h (before the change) use the original ioctl, whether or not the kernel code is actually of the newer version. Thus in most cases there are no compatibility issues.

When can we have compatibility issues? If we have code built with the new dahdi/user.h, but the loaded kernel code (modules) are of the older version. Thus the userspace program will try to use the newer DAHDI_EXAMPLE (0xC008DA3E). But the kernel code has no handler for that ioctl. The result: the error 25, ENOTTY, which means "Inappropriate ioctl for device".

As a by-product of that method, for each interface change a new #define is added. That definition is for the old version and thus it might appear slightly confusing in the code, but it is useful for writing code that works with all versions of DAHDI.

4.13.1. Past Incompatibilities

DAHDI 2.3:

DAHDI_SPANINFO_V1 (extra members added). This will typically only be used on ISDN (PRI/BRI) spans in Asterisk.

DAHDI 2.2:
  • DAHDI_GET_PARAMS_V1, DAHDI_GETCONF_V1, DAHDI_SETCONF_V1, DAHDI_GETGAINS_V1 (direction changed from R to RW to fix FreeBSD support).

  • DAHDI_CONFDIAG_V1, DAHDI_CHANDIAG_V1 (fixed direction).

4.14. Alarm Types

An alarm indicates that a port is not available for some reason. Thus it is probably not a good idea to try to call out through it.

4.14.1. Red Alarm

Your T1/E1 port will go into red alarm when it cannot maintain synchronization with the remote switch. A red alarm typically indicates either a physical wiring problem, loss of connectivity, or a framing and/or line-coding mismatch with the remote switch. When your T1/E1 port loses sync, it will transmit a yellow alarm to the remote switch to indicate that it’s having a problem receiving signal from the remote switch.

The easy way to remember this is that the R in red stands for "right here" and "receive"… indicating that we’re having a problem right here receiving the signal from the remote switch.

4.14.2. Yellow Alarm

(RAI — Remote Alarm Indication)

Your T1/E1 port will go into yellow alarm when it receives a signal from the remote switch that the port on that remote switch is in red alarm. This essentially means that the remote switch is not able to maintain sync with you, or is not receiving your transmission.

The easy way to remember this is that the Y in yellow stands for "yonder"… indicating that the remote switch (over yonder) isn’t able to see what you’re sending.

4.14.3. Blue Alarm

(AIS — Alarm Indication Signal)

Your T1/E1 port will go into blue alarm when it receives all unframed 1s on all timeslots from the remote switch. This is a special signal to indicate that the remote switch is having problems with its upstream connection. dahdi_tool and Asterisk don’t correctly indicate a blue alarm at this time. The easy way to remember this is that streams are blue, so a blue alarm indicates a problem upstream from the switch you’re connected to.

4.14.4. Recovering from Alarm

TODO: explain.

4.14.5. Loopback

Not really an alarm. Indicates that a span is not available, as the port is in either a local or remote loopback mode.

4.14.6. Not Open

Something is not connected. Used by e.g. the drivers of the Astribank to indicate a span that belongs to a device that has been disconnected but is still being used by userspace programs and thus can’t e destroyed.

5. License

This package is distributed under the terms of the GNU General Public License Version 2, except for some components which are distributed under the terms of the GNU Lesser General Public License Version 2.1. Both licenses are included in this directory, and each file is clearly marked as to which license applies.

If you wish to use the DAHDI drivers in an application for which the license terms are not appropriate (e.g. a proprietary embedded system), licenses under more flexible terms can be readily obtained through Digium, Inc. at reasonable cost.

6. Known Issues

6.1. KB1 does not function when echocancel > 128

KB1 was not designed to function at greater than 128 taps, and if configured this way, will result in the destruction of audio. Ideally DAHDI would return an error when a KB1 echocanceller is configured with greater than 128 taps.

7. Reporting Bugs

Please report bug and patches to the Asterisk bug tracker at in the "DAHDI-linux" category.